Potentially Mineralizable C Research Articles

Elucidating the impact of boron fertilization on soil physico-chemical and biological entities under cauliflower-cowpea-okra cropping system in an Eastern Himalayan acidic Inceptisol.

Information on the role of boron (B) on soil physico-chemical and biological entities is scarce, and the precise mechanism in soil is still obscure. Present field investigation aimed to assessing the implication of direct and residual effect of graded levels of applied-B on soil biological entities and its concomitant impact on crop productivity. The treatments comprised of five graded levels of B with four replications. To assess the direct effect of B-fertilization, cauliflower was grown as a test crop wherein, B-fertilization was done every year. For assessment of succeeding residual effects of B-fertilization, cowpea and okra were grown as test crops and, B-fertilization was phased out in both crops. The 100% recommended dose of NPK (RDF) along with FYM was uniformly applied to all crops under CCOCS. Results indicated that the direct effect of B had the edge over residual effect of B in affecting soil physico-chemical and biological entities under CCOCS. Amongst the graded levels of B, application of the highest B level (2 kg ha-1) was most prominent in augmenting microbiological pools in soil at different crop growth stages. The order of B treatments in respect of MBC, MBN, and soil respiration at different crop growth stages was 2.0 kg B ha-1 > 1.5 kg B ha-1 > 1.0 kg B ha-1 > 0.5 kg B ha-1 > 0 kg B ha-1, respectively. Moreover, maximum recoveries of potentially mineralizable-C (PMC) and potentially mineralizable-N (PMN) were noticed under 2 kg B ha-1. Analogous trend was recorded in soil microbial populations at different crop growth stages. Similarly, escalating B levels up to 2 kg B ha-1 exhibited significantly greater soil enzymatic activities viz., arylsulphatase (AS), dehydrogenase (DH), fluorescein diacetate (FDA) and phosphomonoesterase (PMA), except urease enzyme (UE) which showed an antagonistic effect of applied-B in soil. Greater geometric mean enzyme activity (GMEA) and soil functional diversity index were recorded under 2 kg B ha-1 in CCOCS, at all crop growth stages over control. The inclusive results indicated that different soil physico-chemical and biological properties CCOCS can be invariably improved by the application of graded levels of B up to 2 kg B ha-1 in an acid Inceptisol.

Reduced tillage with residue retention improves soil labile carbon pools and carbon lability and management indices in a seven-year trial with wheat-mung bean-rice rotation

Soil total organic carbon (TOC) is a composite indicator of soil quality with implications for crop production and the regulation of soil ecosystem services. Research reports on the dynamics of TOC as a consequence of soil management practices in subtropical climatic conditions, where microbial carbon (C) loss is high, are very limited. The objective of our study was to evaluate the impact of seven years of continuous tillage and residue management on soil TOC dynamics (quantitative and qualitative) with respect to lability and stratification under an annual wheat-mung bean-rice cropping sequence. Composite soil samples were collected at 0-15 and 15-30 cm depths from a three-replicate split-plot experiment with tillage treatment as the main plots and crop residue levels as the sub-plots. The tillage treatments included conventional tillage (CT) and strip tillage (ST). Residue levels were high residue level (HR), 30% of the plant height, and low residue level (LR), 15%. In addition to TOC, soil samples were analyzed for particulate organic C (POC), permanganate oxidizable C (POXC), basal respiration (BR), specific maintenance respiration rate (qCO2), microbial biomass C (MBC), potentially mineralizable C (PMC), and TOC lability and management indices. The ST treatment significantly increased the TOC and labile C pools at both depths compared with the CT treatment, with the effect being more pronounced in the surface layer. The HR treatment increased TOC and labile C pools compared with the LR treatment. The ST + HR treatment showed significant increases in MBC, metabolic quotient (qR), C pool index (CPI), C lability index (CLI), and C management index (CMI), indicating improved and efficient soil biological activities in such systems compared with the CT treatment. Similarly, the stratification values, a measure of soil quality improvement, for POC and MBC were > 2, indicating improved soil quality in the ST + HR treatment compared with the CT treatment. The ST + HR treatment not only significantly increased the contents of TOC pools, but also their stocks. The CMI was correlated with qCO2, BR, and MBC, suggesting that these are sensitive indicators of early changes in TOC. The qCO2 was significantly higher in the CT + LR treatment and negatively correlated with MBC and CMI, indicating a biologically stressed soil condition in this treatment. Our findings highlight that medium-term reduced tillage with HR management has profound consequences on soil TOC quality and dynamics as mediated by alterations in labile C pools.

Grass Buffer Strips Improve Soil Health and Mitigate Greenhouse Gas Emissions in Center-Pivot Irrigated Cropping Systems

Declining water resources and soil degradation have significantly affected agricultural sustainability across the world. In the southern High Plains of USA, buffer strips of perennial grasses alternating with cultivated corn strips were introduced in center-pivot irrigated crop fields to increase agronomic production and ecosystem services. A study was conducted to evaluate soil carbon (C) and nitrogen (N) dynamics, greenhouse gas (GHG) emissions, and soil health benefits of integrating circular grass buffer strips in the center-pivot irrigated corn production system. Multiple parameters were assessed in the grass buffer strips, and at distances of 1.52, 4.57, and 9.14 m away from the edges of grass strips in corn strips. While grasses in the buffer strips depleted N compared to corn strips, potential C mineralization (PCM) was 52.5% to 99.9% more in grass strips than in corn strips. Soil microbial biomass C (MBC) content was 36.7% to 52.5% greater in grass strips than in corn strips. Grass buffer also reduced carbon dioxide (CO2) and nitrous oxide (N2O) emissions from corn strips. Grass buffer strips can improve soil health and sustainability in center-pivot irrigated cropping systems by increasing soil C components and reducing GHG emissions.

Soil health recovery after grassland reestablishment on cropland: The effects of time and topographic position

AbstractThe Conservation Reserve Program (CRP) is a U.S. federal land conservation program that incentivizes grassland reestablishment on marginal lands. Although this program has many environmental benefits, two critical questions remain: does reestablishing grasslands via CRP also result in soil health recovery, and what parts of restored fields (i.e., topographic positions) recover the fastest? We hypothesized that soil health will recover over time after converting cropland to CRP grassland and that recovery will be greatest at higher topographic positions. To test this, we sampled 241 midwestern U.S. soils along a grassland chronosequence (0–40 yr, including native grasslands) and at four topographic positions (i.e., a chronotoposequence). Soils were measured for bulk density, maximum water holding capacity (MWHC), soil organic C (SOC), extractable inorganic N, potentially mineralizable C (PMC), and N. Native grasslands had superior soil health compared with cropland and most CRP soils, and even 40 yr since grassland reestablishment was not adequate for full soil health recovery. Topographic position strongly influenced soil health indicators and often masked any CRP effect, especially with MWHC and SOC. However, PMC (a measure of active C) responded most rapidly to CRP and consistently across the landscape and was 26–34% greater 19–40 yr after grassland reestablishment. Reestablishing grasslands through CRP can improve soil health, although topographic position regulates the recovery, with greatest improvements at shoulder slope positions. Patience is needed to observe changes in soil health, even in response to a drastic management change like conversion of cropland to CRP grassland.

Soil organic matter, greenhouse gas emissions, and sorghum yield in semi‐arid drylands

AbstractSorghum [Sorghum bicolor (L.) Moench] serves as a low‐cost alternative to corn (Zea mays L.) in semi‐arid regions of the world because of its high N and water use efficiencies. However, there has been a concern regarding N loss to the atmosphere as nitrous oxide (N2O) from semi‐arid drylands. This study investigated various soil C and N components, including CO2 and N2O emissions, and crop yield with a dairy compost (13.5 Mg ha−1) and four rates of chemical N fertilizer (0, 22.4, 44.8, and 67.3 kg ha−1) in dryland sorghum. There was no significant difference in soil C and N fractions among N fertilizer rates, although compost addition numerically increased soil C storage and 67.3 kg ha−1 N rate resulted the highest yield in both years. Potential nitrogen mineralization (PNM) was negatively related to crop yield and positively related to grain N content. Soils with greater inorganic N and PNM had a lower carbon dioxide (CO2) emissions, while soils with greater potential C mineralization (PCM) had lower N2O emissions. The results of this study show no significant improvements in yield of dryland sorghum in the semi‐arid environment of southern Great Plains in the short term. However, compost and 44.8 kg N ha−1 applications appeared to be beneficial when both yield and quality were compared.

Small soil C cycle responses to three years of cover crops in maize cropping systems

Cover crops are touted for their ability to improve many ecosystem functions in annual cropping systems. In addition to water and nutrient retention, cover crops may influence C cycling by increasing total C inputs to the agroecosystem, stimulating microbial populations, altering main crop residue decomposition rate, or changing litter chemistry over time. We assessed whether annual (rye) or perennial (bluegrass) cover crops in maize cropping systems influenced maize residue decomposition (litterbags) or microbial communities (shotgun metagenomics) in soil and litter, and whether these cover crops had an effect on microbially active pools of C: particulate organic matter (POM) C and N, or potentially mineralizable C (PMC) after three years of cover crops. Neither cover crop affected litterbag decay rates or microbial composition relative to no cover crop controls. However, both cover crop types increased PMC indicating that microbially-available C was boosted by cover crops. Total POM and POM-N were higher with bluegrass cover crops. These modest effects of cover crops on dynamic soil C pools suggest the generally positive long-term effects of cover crops on soil protection and nutrient retention are related to incremental shifts in quantity, timing and quality of C inputs.

Minimal soil disturbance and increased residue retention increase soil carbon in rice-based cropping systems on the Eastern Gangetic Plain

The adoption of conservation agriculture (CA) in the intensive triple-cropping, rice-based systems of the Eastern Gangetic Plain (EGP) alters the dynamics of carbon (C) in the soil, but the nature of these changes is poorly understood. Our aim was to determine whether CA in these systems involving non-puddled transplanting of wetland rice and strip planting of dryland crops plus increased residue retention would increase the C storage in soils relative to conventional crop establishment practices. Long-term field experiments were studied in two locations of northwestern Bangladesh to determine C turnover as well as examining C cycling under three levels of soil disturbance (conventional tillage (CT), strip planting (SP) and bed planting (BP)) in combination with low residue (straw) retention (LR, the current practice) and increased residue retention (HR) in Calcareous Brown Floodplain soil (Alipur) and Grey Terrace soil (Digram). The total nitrogen (N), organic C, microbial biomass C (MBC) and water-soluble C (WSC) values were measured in soil samples from 0 to 10 cm depth collected at different stages during the growth of the 13th and 14th crops at Alipur and the 12th and 13th crops at Digram since the treatments commenced. At each location, SP and BP with either LR or HR retained more soil organic C (0–10 cm) from C inputs than CT with HR and LR. In general, the CO2 emissions relative to the stored soil organic C in the soils (0–10 cm) under SP with LR and HR were approximately 13 to 59% lower than those under CT and BP with LR and HR. The higher levels of C mineralization were associated with higher WSC contents in the soil. In contrast, the MBC contents in the HR treatments followed the order SPHR > BPHR > CTHR. Similarly, in SPLR and SPHR, the potentially mineralizable C (PMC) was higher, while the decay rate constant was lower. Increased residue retention with minimal soil disturbance practices (SP and non-puddled transplanting) after 14 crops at Alipur and 13 crops at Digram modified the C cycle by decreasing C emissions and increasing the levels of total organic C in the soil. The application of both minimal soil disturbance and increased residue retention enhanced soil organic C (0–10 cm) concentrations in the two soils under intensive rice-based cropping systems on the EGP.

Soil carbon fractions in response to straw mulching in the Loess Plateau of China

Straw mulching has been used to conserve soil water and sustain dryland crop yields, but the impact of the quantity and time of mulching on soil C fractions are not well documented. We studied the effects of various amounts and times of wheat (Triticum aestivum L.) straw mulching on soil C fractions at 0–10- and 10–20-cm depths from 2009 to 2017 in the Loess Plateau of China. Treatments were no mulching (CK), straw mulching at 9.0 (HSM) and 4.5 Mg ha−1 (LSM) in the winter wheat growing season, and straw mulching at 9.0 Mg ha−1 in the summer fallow period (FSM). Soil C fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). All C fractions at 0–10 and 10–20 cm were 8–27% greater with HSM and LSM than FSM and CK. Both SOC and POC at 0–10 cm increased at 0.32 and 0.27 Mg ha−1 year−1 with HSM and at 0.40 and 0.30 Mg C ha−1 year−1 with LSM, respectively, from 2009 to 2017. Winter wheat grain yield was lower with HSM and LSM, but total aboveground biomass was greater with HSM than other treatments. All C fractions at most depths were correlated with the estimated wheat root residue returned to the soil and PCM at 0–10 and 0–20 cm was correlated with wheat grain yield. Wheat straw mulching during the growing season increased soil C sequestration and microbial biomass and activity compared with mulching during the fallow period or no mulching, regardless of mulching rate, due to increased C input, although it reduced wheat grain yield. Continuous application of straw mulching over time can increase soil C sequestration by increasing nonlabile C fractions while decreasing labile fractions. Straw mulching at higher rate and mulching during the summer fallow period had no additional benefits in soil C sequestration.

Response of Soil Carbon Fractions and Dryland Maize Yield to Mulching

Core Ideas Root growth due to mulching may enhance soil C fractions and maize yield. Straw and plastic film mulching effect on soil C fractions and maize yield studied. Plastic film mulching increased maize yield, but reduced soil C fractions. Straw mulching enhanced C fractions and maize yield compared with no mulching. Straw mulching can increase C sequestration, soil health, and dryland maize yield. Stimulation of root growth from mulching may enhance soil C fractions under maize (Zea mays L.). We studied the 5‐yr straw (SM) and plastic film (PM) mulching effect on soil C fractions and maize yield compared with no mulching (CK) in the Loess Plateau of China. Soil samples collected from 0‐ to 10‐ and 10‐ to 20‐cm depths after maize harvest in the fall, 2011 to 2015, were analyzed for soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At both depths, all C fractions were 7 to 35% greater with SM than PM and CK. At 0 to 20 cm, SOC increased at 0.87 Mg C ha–1 yr–1 with PM and POC increased at 0.18 and 0.54 Mg C ha–1 yr–1 with SM and PM, respectively, from 2011 to 2015. Maize grain yield and aboveground biomass were 5 to 33% greater with PM and SM than CK. The PCM and MBC at all depths were negatively correlated with maize grain yield, but SOC and POC at 10 to 20 cm were positively correlated with estimated maize root residue returned to the soil. Plastic film mulching increased maize yield, but reduced soil C fractions compared with straw mulching. Because of favorable effect on soil C fractions and maize yield, straw mulching can enhance soil C sequestration by increasing labile and intermediate C fractions and sustain maize yield compared with no mulching under dryland cropping systems.

Soil Carbon Fractions in Response to Long‐Term Crop Rotations in the Loess Plateau of China

Core Ideas Long‐term (30 yr) diversified crop rotations had a positive impact on soil C fractions. Carbon fractions increased with increased rotation length. Diversified rotations enhanced soil C sequestration and microbial biomass and activity. Diversified crop rotations may enhance C fractions and soil quality by affecting the quality and quantity of crop residue returned to the soil compared with monocropping and fallow. We evaluated the effect of 30‐yr‐old diversified crop rotations on soil C fractions at 0‐ to 15‐, 15‐ to 30‐, and 0‐ to 30‐cm depths in the Loess Plateau of China. Crop rotations were continuous winter wheat (Triticum aestivum L.) (W); 3‐yr rotations of corn (Zea mays L.)–winter wheat–winter wheat–millet (Eleusine coracana L.) (CWWM), pea (Pisum sativum L.)–winter wheat–winter wheat–millet (PWWM), and sainfoin (Onobrychis viciifolia Scop.)–winter wheat–winter wheat–sainfoin (SWWS); 4‐yr rotation of pea–winter wheat–winter wheat–corn (PWWC); and 8‐yr rotation of alfalfa (Medicago sativa L.) (4 yr)–potato (Solanum tuberosum L.) (1 yr)–winter wheat (3 yr) (A4PoW3). A fallow (F) treatment was also included for comparison. Carbon fractions were soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At all depths, SOC and POC were greater in A4PoW3 than F, W, and PWWC. The PCM and MBC were greater with SWWS than most other crops rotations. Soil C fractions increased linearly with increased belowground crop residue returned to the soil. Both SOC and POC increased linearly with increased rotation length. All C fractions were also correlated with mean annualized crop grain yield. Diversified crop rotations with increased belowground residue production enhanced soil C sequestration and microbial biomass and activity as well as crop yield compared with monocropping.

Soil microbial community responses to long-term land use intensification in subtropical grazing lands

Land use intensification often results in modification in plant cover and nutrient inputs with subsequent potential effects on composition and structure of soil microbial community and fractions. The objective of this study was to understand the long-term (>22yr) impacts of land use intensification (introduction of productive vegetation type, greater N fertilizer input and stocking rate) on soil microbial community composition and activity in Florida grazing lands. Experimental sites consisted of a gradient of management intensities ranging from native rangeland (lowest), silvopasture (intermediate), to sown pasture (highest). Increasing management intensity from native rangeland to sown promoted microbial biomass and activity. At the 0–10cm, soil microbial biomass carbon (MBC) concentration was greater in sown pasture (334mgkg−1) compared to silvopasture and native rangeland (193 and 232mgkg−1, respectively). Similarly, potentially mineralizable C (PMC) increased in response to grazing land intensification (1.2mg CO2-C kg−1d−1 for sown pastures vs. 0.5 and 0.6mg CO2-C kg−1d−1 for native rangeland and silvopasture, respectively). Sown pastures exhibited the greatest levels of β-glucosidase activity (203nmolg−1 soil h.−1) and phospholipid fatty acid (PLFA) biomass (222μmolkg−1 soil) compared to native rangeland and silvopasture. Results also demonstrated greater relative abundance of bacteria and less fungi as land use intensification increased from native rangeland to silvopasture or sown pasture. Our study indicated that long-term land use intensification affected the size, activity, and composition of soil microbial community in subtropical grazing lands.

Sheep Grazing Enhances Coarse Relative to Microbial Organic Carbon in Dryland Cropping Systems

Sheep (<em>Ovis aries </em>L<em>.</em>) grazing, a cost-effective method of weed control compared with herbicide application and tillage, may influence soil C fractions by consuming crop residue and weeds and returning C through feces and urine to the soil. We examined the effect of three weed management practices (sheep grazing, herbicide application, and tillage) and two cropping sequences (continuous spring wheat [<em>Triticum aestivum </em>L.] [CSW] and spring wheat-pea [<em>Pisum sativum </em>L.]/barley [<em>Hordeum vulgare </em>L.] mixture hay-fallow [W-P/B-F]) on soil microbial biomass C (MBC), potential C mineralization (PCM), and particulate organic C (POC) in relation to soil organic C (SOC) at the 0- to 30-cm depth from 2009 to 2011 in southwestern Montana. The MBC at 0 to 5 cm was greater with tillage on CSW than tillage on W-P/B-F in 2009 and 2011, but was greater with herbicide application on CSW than tillage on CSW in 2010. The POC at 0 to 5 cm and 15 to 30 cm was greater with sheep grazing than herbicide application on CSW and W-P/B-F, but at 5 to 15 cm was greater with grazing on CSW. The MBC, PCM, and POC at all depths decreased from 2009 to 2011. Crop residue incorporation into the soil increased MBC with tillage on CSW. Lower proportions of labile than nonlabile organic matter through feces and urine probably reduced MBC at the soil surface, but increased POC with sheep grazing compared with herbicide application on CSW and W-P/B-F. Sheep grazing may increase coarse soil organic matter compared with microbial biomass in dryland cropping systems.

Soil C turnover, microbial biomass and respiration, and enzymatic activities following rangeland conversion to wheat–alfalfa cropping in a semi-arid climate

Conversion from rangeland to cropland potentially influences soil organic matter (SOM) dynamics and biochemical properties. The purpose of the current study was to assess changes in soil properties and investigate the interaction between SOM and enzyme activities following cultivation of native rangelands for more than 40 years. The quantities of soil organic carbon (C) and microbial biomass C (MBC), aggregate stability, microbial respiration rate (MRR), potential C mineralization (PCM) and the activities of soil enzymes involved in C, N, P and S cycling at the 0–20 and 20–40 cm depths were evaluated in never-cultivated natural rangelands and their corresponding farmlands with similar soil type and climatic conditions at Dehnow located in Central west, Iran. Cultivation in native rangelands resulted in a significant decline of most soil properties in farmlands, depending on soil sampling depths. Conversion of rangelands to their farmland counterparts decreased soil C (50–66 %), MBC (48–61 %), MRR (20–44 %), PCM (45–59 %), the activities of urease (5–17 %), invertase (7–28 %) and arylsulfatase (17–20 %) per soil mass, while increased C turnover rate (61–63 %), microbial metabolic quotient (qCO2) values (41–54 %) and enzyme activities per unit C and MBC (61–190 %) without affecting microbial quotient (MBC/C ratio) values and the activities of soil phosphatases per soil mass. The decreases in soil microbial and biochemical properties were largely the result of a substantial decline in soil C contents and aggregate stability by tillage activities in farmlands. Although the absolute activities of soil enzymes (i.e., activity per dry soil mass) showed inconsistent responses to land use changes and were even less responsive than soil organic C and MBC contents to changing land uses, the specific enzyme activities expressed either per unit of C or MBC tended to increase considerably and consistently with this conversion and were more sensitive than soil C and MBC contents. Soil C turnover rates and metabolic quotients were found to correlate positively with the specific enzyme activities, indicating an interaction between SOM and enzyme activities. The specific enzymatic activity could be an appropriate and integrative simple index to (1) assess soil quality changes, (2) detect changes in soil microbial community; and even (3) associate the losses of SOM with the presence of metabolically more active decomposer biota in rangelands cultivated for the long term.

Labile soil organic carbon and nitrogen within a gradient of dryland agricultural land-use intensity in Wyoming, USA

Tillage in dryland winter wheat (Triticum aestivum L.)–summer fallow cropping systems of the central High Plains, USA, has caused significant erosion and loss of soil organic matter (SOM), underscoring the need for more sustainable practices on marginally-productive semiarid lands. Conversion to no-till (NT) wheat–fallow or perennial grass–legume cover, as in the Conservation Reserve Program (CRP), provides viable alternatives to tillage-based wheat–fallow practices. However, the overall impact of transitioning on soil biogeochemistry is not fully understood. Labile SOM pools were determined on soil cores (0–120cm) collected in July 2011 from five unfertilized fields that spanned a gradient in disturbance associated with land-use practices: a more intensively tilled wheat–fallow (‘historic’, HT) than a conventional (CT) wheat–fallow with limited herbicides combined with tillage, NT wheat–fallow that exclusively used herbicides for ≥10years, formerly cultivated HT wheat–fallow planted to grass–legume mixture as in the CRP for 7years, and native prairie (NP) with no history of agricultural disturbance. Significant differences in microbial biomass C (MBC), potentially mineralizable C (PMC) and nitrogen (PMN) were largely confined to the surface soil (0–30cm), following the order NP>CRP>all wheat–fallow systems. When expressed on a per soil organic C (SOC) basis, both MBC and PMC followed the order CRP>NP>all wheat–fallow systems. In contrast, when normalized by total soil N (TN), PMN was higher in HT and CT soils than in other soils (NT, CRP, and NP). MBC and PMC were positively correlated with SOC whereas PMN, soil pH, and electrical conductivity (EC) showed a negative relationship with SOC. These results suggest more efficient conservation of SOM under perennial grass–legume system than NT wheat–fallow rotation. We suggest that cessation of tillage alone may not be sufficient for the recovery of labile-pool SOM degraded through long-term cultivation in the absence of inputs for soil fertility renewal.

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions

Information is needed on novel management practices to increase dryland C sequestration and soil quality in the northern Great Plains, USA. We evaluated the effects of tillage, crop rotation, and cultural practice on dryland crop biomass (stems and leaves) yield, surface residue, and soil C fractions at the 0-20 cm depth from 2004 to 2008 in a Williams loam in eastern Montana, USA. Treatments were two tillage (no-tillage [NT] and conventional tillage [CT]), two crop rotations (continuous spring wheat [Triticum aestivum L.] [CW] and spring wheat-barley [Hordeum vulgaris L.] hay-corn [Zea mays L.]-pea [Pisum sativum L.] [W-B-C-P]), and two cultural practices (regular [conventional seed rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height] and ecological [variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height]). Carbon fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Crop biomass was 24% to 39% greater in W-B-C-P than in CW in 2004 and 2005. Surface residue C was 36% greater in NT than in CT in the regular practice. At 5 - 20 cm, SOC was 14% greater in NT with W-B-C-P and the regular practice than in CT with CW and the ecological practice. In 2007, POC and PCM at 0 - 20 cm were 23 to 54% greater in NT with CW or the regular practice than in CT with CW or the ecological practice. Similarly, MBC at 10 - 20 cm was 70% greater with the regular than with the ecological practice in NT with CW. Surface residue, PCM, and MBC declined from autumn 2007 to spring 2008. No-tillage with the regular cultural practice increased surface residue and soil C storage and microbial biomass and activity compared to conventional tillage with the ecological practice. Mineralization reduced surface residue and soil labile C fractions from autumn to spring.

Dryland soil carbon dynamics under alfalfa and durum-forage cropping sequences

Forages grown in rotation with or without cereals to sustain dryland soil water content and crop production may influence C dynamics. We evaluated the effect of alfalfa ( Medicago sativa L.) and durum ( Triticum turgidum L.)-annual forage cropping sequences on above- (stems + leaves) and belowground (roots) biomass C inputs and dryland soil organic C (SOC), particulate total C (PTC), microbial biomass C (MBC), and potential C mineralization (PCM) at the 0–120 cm depth. Cropping sequences were continuous alfalfa (CA), durum-barley ( Hordeum vulgare L.) hay (D-B), durum-foxtail millet ( Setaria italica L.) hay (D-M), durum-Austrian winter pea ( Pisum sativum L.)/barley mixture hay (D-P/B), and durum-fallow (D-F). The experiment was conducted in a Williams loam (fine-loamy, mixed, superactive, Typic Argiustoll) from 2002 to 2005 in eastern Montana, USA. Except in 2003, aboveground biomass yield and C content were lower in CA than in other treatments from 2002 to 2005. Similarly, belowground biomass yield and C content were lower in D-F than in other treatments from 2003 to 2005. In 2005, soil surface residue amount and C content were greater in D-F than in other treatments. The SOC at 0–15 cm was greater in CA than in D-B and D-M. The PTC at 0–15 cm was greater in CA than in other treatments, but varied with treatments at other depths. The PCM at 0–120 cm was greater in CA than in other treatments. The MBC at 30–120 cm was greater in CA and D-P/B than in D-B. The proportion of SOC in PTC, PCM, and MBC at 0–120 cm was greater in CA or D-P/B than in D-B and D-F. Although aboveground biomass C input was lower, greater belowground biomass C and a relatively undisturbed soil condition probably increased C storage at the surface layer and microbial biomass and activity at the surface and subsurface layers under alfalfa than under annual durum-forage sequences. Greater aboveground biomass C and fresh residue accumulation during durum phase, however, increased C storage in the surface residue under durum-fallow than under other cropping sequences. Perennial forages, such as alfalfa, can increase dryland soil C sequestration and biological soil quality by increasing microbial biomass and activity compared with annual cropping systems due to greater belowground biomass C input and continuous root growth.

Tillage, Cropping Systems, and Nitrogen Fertilizer Source Effects on Soil Carbon Sequestration and Fractions

Quantification of soil carbon (C) cycling as influenced by management practices is needed for C sequestration and soil quality improvement. We evaluated the 10-yr effects of tillage, cropping system, and N source on crop residue and soil C fractions at 0- to 20-cm depth in Decatur silt loam (clayey, kaolinitic, thermic, Typic Paleudults) in northern Alabama, USA. Treatments were incomplete factorial combinations of three tillage practices (no-till [NT], mulch till [MT], and conventional till [CT]), two cropping systems (cotton [Gossypium hirsutum L.]-cotton-corn [Zea mays L.] and rye [Secale cereale L.]/cotton-rye/cotton-corn), and two N fertilization sources and rates (0 and 100 kg N ha(-1) from NH(4)NO(3) and 100 and 200 kg N ha(-1) from poultry litter). Carbon fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Crop residue varied among treatments and years and total residue from 1997 to 2005 was greater in rye/cotton-rye/cotton-corn than in cotton-cotton-corn and greater with NH(4)NO(3) than with poultry litter at 100 kg N ha(-1). The SOC content at 0 to 20 cm after 10 yr was greater with poultry litter than with NH(4)NO(3) in NT and CT, resulting in a C sequestration rate of 510 kg C ha(-1) yr(-1) with poultry litter compared with -120 to 147 kg C ha(-1) yr(-1) with NH(4)NO(3). Poultry litter also increased PCM and MBC compared with NH(4)NO(3). Cropping increased SOC, POC, and PCM compared with fallow in NT. Long-term poultry litter application or continuous cropping increased soil C storage and microbial biomass and activity compared with inorganic N fertilization or fallow, indicating that these management practices can sequester C, offset atmospheric CO(2) levels, and improve soil and environmental quality.

Long‐Term Tillage and Cropping Sequence Effects on Dryland Residue and Soil Carbon Fractions

Long‐term management practices are needed to increase dryland C storage and improve soil quality. We evaluated the 21‐yr effects of combinations of tillage and cropping sequences on dryland crop biomass (stems + leaves) returned to the soil, residue C, and soil C fractions at the 0‐ to 20‐cm depth in a Dooley sandy loam (fine‐loamy, mixed, frigid, Typic Argiborolls) in eastern Montana. Treatments were no‐till continuous spring wheat (Triticum aestivum L.) (NTCW), spring‐tilled continuous spring wheat (STCW), fall‐ and spring‐tilled continuous spring wheat (FSTCW), fall‐ and spring‐tilled spring wheat–barley (Hordeum vulgare L.) (1984–1999) followed by spring wheat–pea (Pisum sativum L.) (2000–2004) (FSTW‐B/P), and spring‐tilled spring wheat–fallow (STW‐F). Carbon fractions were soil organic C (SOC), soil inorganic C (SIC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Mean crop biomass was 53 to 66% greater in NTCW, STCW, FSTCW, and FSTW‐B/P than in STW‐F. Soil surface residue amount and C content in 2004 were 46 to 60% greater in NTCW and FSTCW than in STW‐F. As a result, soil C fractions at 0 to 20 cm were 23 to 141% greater in all other treatments than in STW‐F due to increased C input. At 0 to 5 cm, SOC, SIC, POC, and PCM were greater in NTCW than in FSTW‐B/P. At 5 to 20 cm, POC was greater in NTCW than in FSTW‐B/P and PCM was greater in STCW than in FSTCW. Long‐term reduced tillage with continuous nonlegume cropping increased dryland crop biomass, residue and soil C storage, and soil quality by increasing microbial biomass and activities compared with a conventional system such as STW‐F.

Cover crop effect on soil carbon fractions under conservation tillage cotton

Cover crops may influence soil carbon (C) sequestration and microbial biomass and activities by providing additional residue C to soil. We examined the influence of legume [crimson clover ( Trifolium incarnatum L.)], nonlegume [rye ( Secale cereale L.)], blend [a mixture of legumes containing balansa clover ( Trifolium michelianum Savi), hairy vetch ( Vicia villosa Roth), and crimson clover], and rye + blend mixture cover crops on soil C fractions at the 0–150 mm depth from 2001 to 2003. Active fractions of soil C included potential C mineralization (PCM) and microbial biomass C (MBC) and slow fraction as soil organic C (SOC). Experiments were conducted in Dothan sandy loam (fine-loamy, kaolinitic, thermic, Plinthic Kandiudults) under dryland cotton ( Gossypium hirsutum L.) in central Georgia and in Tifton loamy sand (fine-loamy, siliceous, thermic, Plinthic Kandiudults) under irrigated cotton in southern Georgia, USA. Both dryland and irrigated cotton were planted in strip tillage system where planting rows were tilled, thereby leaving the areas between rows untilled. Total aboveground cover crop and cotton C in dryland and irrigated conditions were 0.72–2.90 Mg C ha −1 greater in rye + blend than in other cover crops in 2001 but was 1.15–2.24 Mg C ha −1 greater in rye than in blend and rye + blend in 2002. In dryland cotton, PCM at 50–150 mm was greater in June 2001 and 2002 than in January 2003 but MBC at 0–150 mm was greater in January 2003 than in June 2001. In irrigated cotton, SOC at 0–150 mm was greater with rye + blend than with crimson clover and at 0–50 mm was greater in March than in December 2002. The PCM at 0–50 and 0–150 mm was greater with blend and crimson clover than with rye in April 2001 and was greater with crimson clover than with rye and rye + blend in March 2002. The MBC at 0–50 mm was greater with rye than with blend and crimson clover in April 2001 and was greater with rye, blend, and rye + blend than with crimson clover in March 2002. As a result, PCM decreased by 21–24 g CO 2–C ha −1 d −1 but MBC increased by 90–224 g CO 2–C ha −1 d −1 from June 2001 to January 2003 in dryland cotton. In irrigated cotton, SOC decreased by 0.1–1.1 kg C ha −1 d −1, and PCM decreased by 10 g CO 2–C ha −1 d −1 with rye to 79 g CO 2–C ha −1 d −1 with blend, but MBC increased by 13 g CO 2–C ha −1 d −1 with blend to 120 g CO 2–C ha −1 d −1 with crimson clover from April 2001 to December 2002. Soil active C fractions varied between seasons due to differences in temperature, water content, and substrate availability in dryland cotton, regardless of cover crops. In irrigated cotton, increase in crop C input with legume + nonlegume treatment increased soil C storage and microbial biomass but lower C/N ratio of legume cover crops increased C mineralization and microbial activities in the spring.

Soil Carbon Dioxide Flux and Organic Carbon in Grassland after Manure and Ammonium Nitrate Application

Fertilization effects on changes in soil flux and organic C in switchgrass (Panicum virgatum L.) land managed for biomass production were investigated. The mean daily soil flux in the manure treatment was 5.63 g , and this was significantly higher than the mean value of 3.36 g in the control. The mean daily fluxes in N and P fertilizer treatments plots were not different when compared to the value in the control plots. Potentially mineralizable C (PMC), soil microbial biomass C (SMBC), and particulate organic C (POC) were highest at the 0 to 10 cm depth of the manure treatment. Potentially mineralizable C had the strongest correlation with SMBC (r = 0.91) and POC (r = 0.84). There was also a strong correlation between SMBC and POC (r = 0.90). Our results indicated that for the N and P levels studied, fertilization had no impact on temporal changes in soil organic C, but manure application had a significant impact on temporal changes in soil evolution and active C constituents such as PMC, SMBC, and POC.